In the 1870s, the Osage Indians were forced off their land in Kansas — land they had been forced onto just a few decades earlier. This time, they were sent to live on a rocky, arid reservation in Oklahoma. The new land was barren, the buffalo had been depleted, and the Indians began to starve.

And then someone discovered oil.

Money poured in. Prospectors paid the Osage for oil leases and royalties, and by the start of the 20th century every member of the tribe was receiving healthy quarterly checks. They built magnificent houses, bought cars, hired servants. Suddenly, the Oklahoma Osage were among the richest people in the world.

But wherever there are Indians with anything — land, animals and, especially, money and oil rights — there are white people ready to fleece them. David Grann’s “Killers of the Flower Moon” tells the horrifying story of mass murder in Gray Horse, Okla. Over a period of five years in the 1920s, more than two dozen Osage Indians were killed — poisoned, shot, blown up, hit by cars — for their money and oil rights.

“Flower Moon” opens with the feel of an Erik Larson book — “Devil in the White City,” perhaps: an entertaining murder mystery set in the historical context of 100 years ago.

But Grann’s book quickly grows darker, and then darker still. It is superbly done — meticulously researched, well-written — but it is hard to be entertained by a story of such unmitigated evil.

When the oil money started flowing, the U.S. government quickly decided that Indians could not possibly be capable of managing their own finances. Many of them were assigned white guardians, “overseeing and authorizing all of their spending, down to the toothpaste they purchased at the corner store,” Grann writes. “The guardians were usually drawn from the ranks of the most prominent white citizens in Osage County.”

Prominent, yes. But not necessarily trustworthy. Guardians skimmed off millions of dollars from the Indians whose wealth they were supposed to manage. Merchants in Gray Horse jacked prices way up for the Indians, but not for white citizens.

But all of this pales next to the murders.

In the early 1920s, Osage Indians began dying in so many unusual ways and in such high numbers that the tribe dubbed it the Siege of Terror.

Grann follows the fate of Mollie Burkhart, an Osage woman married to a white man. Mollie’s sister Minnie died of a mysterious “wasting illness.” Her sister Anna was shot in the head. Her mother died of that same wasting illness. And then Mollie fell deathly ill and barricaded herself in her house “in dread, knowing that she was the likely next target in the apparent plot to eliminate her family.”

No one seemed able to solve any of the murders. Evidence went missing; witnesses ended up dead — one of them was stripped naked and thrown off a moving train.

This steady string of murders eventually attracted the attention of Washington, and a young J. Edgar Hoover. He had been looking for ways to bolster the profile and powers of his Bureau of Investigation, which, at that time, investigated crimes but could not make arrests.

He dispatched Tom White, a former Texas Ranger, to Oklahoma. There, White “quickly found himself wandering through a wilderness of mirrors — his work more akin to espionage than to criminal investigation. There were moles and double agents and possibly triple agents.”

Eventually, White uncovered an enormous conspiracy masterminded by a single person who was “a connoisseur of plots … calculating enough to carry out his diabolical vision over years.”

Grann digs deep. He spent years on the research, examining FBI files, court testimony, private correspondence, field reports from the Bureau of Indian Affairs, diary entries and scores of other documents.

The result is a powerful book — not entertaining, no, but fascinating; an outrageous, devastating read.

Laurie Hertzel is the senior editor for books at the Star Tribune. On Twitter: @StribBooks.